The fabrication of silicon-based photovoltaic solar cells from thin silicon wafers, typically 140-180 micrometers thick, requires multiple processing steps, including a 2-stage diffusion process to create a semi-conducting “p-n”, junction diode layer, followed by screen-printing “solder paste” coatings on the wafer front and back sides which are fired into the p-n junction or back contact layer, where they act as ohmic collectors and grounds, respectively.
The diffusion process includes coating the wafer with a phosphoric or/and boric acid composition, followed by firing in a diffusion furnace to create a P-doped p/n junction photovoltaic layer on the front side, or/and a B-doped contact layer on the back side. After diffusion and various cleaning steps, the wafers are coated with an Anti-Reflective Coating (ARC), typically silicon nitride (SiN3) which renders the wafers deep blue or brown, depending on the ARC coating used.
To form a back contact ground layer, the wafer back surface is coated with an Al-based paste. The wafer top surface is screen printed with a fine network of Ag-based paste lines connected to larger buss conductors to “collect” the electrons generated. After these pastes have been dried, they are “co-fired” at high temperature in an IR lamp-heated conveyor-type metallization furnace.
Currently available IR conveyor furnaces for such processing steps are single line, that is a single conveyor belt or roller system that conveys the wafers through the processing step, single file. All wafers are processed according to the same processing schedule and dwell time in each processing zone. To double production requires buying and installing a second line of a multiplicity of modules arranged end-to-end. Each module has its own drive, its own transport system, its own framework including sides top and bottom, and requires the same factory floor-space foot print. To double production requires double the factory real estate and double the capital equipment for the processing machinery and the operating personnel.
For example, in the case of diffusion firing processes, the furnaces have a long heating chamber in which a plurality of IR lamps are substantially evenly spaced apart (typically 1.5″ apart) both above and below the wafer transport system (wire mesh belt or ceramic roller conveyor). The heating zone is insulated from the outside environment with various forms of insulation, compressed insulating fiber board being the most common. The infra-red (IR) lamps increase the temperature of the incoming silicon wafers to approximately 700° C. to 950° C. This temperature is held for the 30-minute duration of the diffusion process, after which the wafers are cooled and transferred to the next downstream process operation and equipment.
Currently available conveyor-type liquid dopers (as distinct from the muffle tube and carrier-type POCl3 gas dopers) employ solid or elastomeric band conveyors on which the wafers travel. The wafers are adhered to a peel-off disposable paper backing to protect the wafer back side against doping chemical exposure. These are non-conductive conveyor systems, which involve the extra step of wafer handling to remove the paper backing.
Currently available diffusion furnaces typically employ one of two types of wafer transport systems: 1) a plurality of static (not-longitudinally moving), solid ceramic, rotating rollers; or 2) active (longitudinally moving) wire mesh belts, to convey the wafers through the furnace firing zone. Static, ceramic rotating-roller furnaces currently are preferred in order to minimize or prevent metallic contamination of the back surface of the wafers.
A typical conventional diffusion furnace is on the order of 400″ long, having 160, 36″-wide IR lamps placed above the rollers, with from 100-160 placed below. The total mass of the conveyor rollers is on the order of 800 lbs, and is classified as a high-mass conveyor system.
As the demand for solar cells increases, the rates of production must increase, either by process improvements or adding furnaces into service. With respect to adding furnaces, conventional furnaces have a large footprint. Thus, adding furnaces requires increased capital outlay, for buildings, the furnaces themselves, and related service facilities.
In the case of wire mesh belts used in the metallization furnaces, the mesh belts must be supported beneath the belts to prevent sagging. Long-standing practice in the industry is to provide supports comprising pairs of opaque, white quartz tubes, typically on the order of 2-3 cm in diameter, placed with their long axes parallel-to or slightly canted to the direction of travel of the belt, e.g. in a staggered converging or diverging (herringbone) pattern. The quartz tubes are smooth, and provide line contact surfaces on which the underside of the belt slides as it conveys the printed wafers through the metallization furnace processing zones. To minimize shadowing by the tubes, the practice has long been to angle the tubes, either converging or diverging along the line of travel so that the same portion of the wafer was not shadowed the entire duration of travel through the furnace. The shadow effect is reduced by this long-used trick of angling the support tubes, but not eliminated, because now the entire wafer is in shadow at least some of the duration of transport through the furnace. In effect, the shadow lines are there, less pronounced and more diffuse, but broader.
In addition, the contact of the back side of the wafer with the many wires of the conveyor belt contributes to abrasion and contamination of the back contact layer paste during the metallization firing process. In an attempt to minimize this problem, current conveyor belts employ “pips”, which raise the wafer a few millimeters above the belt. The pips are made by bending a plurality of loops of the wire mesh belt upward of the top plane of the belt. However, the wafer bottom still rests directly on the pips, on the order of 10-20 per wafer, and they still move laterally and forward or back on the order of a millimeter in each direction during the transport of the wafers through the processing zones. This results in reduced throughput, due to discarding pip-damaged and contaminated wafers
Thus, the need for faster production and greater throughput, while curbing facility capital outlay, is not met by the current state of the art quartz tube-supported metal belts having wafer support pips. In order to compensate, conveyor-type dopers and furnaces have been made laterally wider, so that multiple lines of wafers can be processed in each process zone. In the case of furnaces, this in turn requires longer, more expensive lamps which typically experience a substantially shorter mean time to failure, thus significantly increasing operating costs.
Since there are dimensional and IR lamp cost constraints, increasing lamp density in the furnace is not generally a feasible solution. Likewise, increasing the power to the lamps is not currently feasible because higher output can result in overheating of the lamp elements, as a result of the thermal mass of the furnace, principally in the high mass solid ceramic roller conveyor system. Overheating particularly affects the external quartz tubes of the lamps. Most furnaces are thermocouple controlled. Since the IR lamps are placed side by side, on the order of 1.25″ apart, each lamp heats lamps adjacent to it. When the thermocouples detect temperatures approaching the selected diffusion or sintering temperature set point in the 700-950° C. range, they automatically cut back power to the lamps by an amount that depends on the thermal mass of the transport system (rollers or metal mesh belts and quartz tube supports). This lower power density is accompanied by substantial changes in the spectral output of the IR lamp emissions (hence a lower light flux and energy output). In turn, this reduced light flux results in the need to slow down the conveyor belt speed or lengthen the furnace (while maintaining the original belt speed), thus slowing processing. Overheating of lamps, e.g., due to thermocouple delay or failure, can cause the lamps to deform, sag and eventually fail. Lamp deformation also affects uniformity of IR output delivered to the wafers.
Accordingly, there is an unmet need in the wafer processing art to increase production at costs that are less than the unit cost of duplication of process lines. In addition, in the diffusion to and metallization furnace and firing process art there is an unmet need to significantly improve net effective use of firing zone(s) by reduction in wafer pip damage and contamination, permit-ting improved utilization of firing energy, improving the speed and uniformity of the firing process, reducing furnace size while retaining or improving throughput, and accomplishing these goals on a reduced furnace footprint, and lower energy, operating and maintenance costs.